Abstract

Plasmonic inverse-rib optical waveguides, consisting of a high-index inverse rib embedded in low-index medium above a flat metallic surface, are investigated under four aspects: (i) the optimal angle θ of the rib sidewall for tight modal confinement is assessed, (ii) the effect of the geometric parameters and the wavelength on propagation losses is given, (iii) we use a 3D simulation to assess how well light from an emitting dipole is captured by such a tightly guiding structure, and (iv) we show that for two such parallel hybrid waveguiding systems, when one of them has added gain, we have a plasmonic version of the PT-symmetric waveguide arrangement, and we additionally show that complex gain is needed to restore a truly exceptional point in its propagation constant evolution.

Losses as a function of wavelength for a 45° PIROW and a 15° PIROW as indicated, using gold. Inset, the associated absorption lengths that become larger than a typical penetration length in a periodic system of given modulation Δneff at wavelengths between 600 and 700 nm.

(a) Effective index versus wavelength for a 45° PIROW with Au (top curve) or Ag (lower curve) and (b) same as Fig. 5, but comparing Ag (dashed line) and Au (solid line) losses Im(neff). The inset transforms the same comparison into an absorption length one, as in Fig. 5. Note that Ag may behave well at wavelengths as short as 550–600 nm.

Purcell factor FP for a vertical dipole at wavelength λ=700nm in a PIROW: (a) dipole at variable height, either in the symmetry plane x=0 or along the rib edge and (b) dipole lying along x at height z=d, the height of the tip bottom. Insets, scanned positions.

Light capture efficiency by a PIROW on both sides for a dipole lying along the vertical direction at x=0. Insets show (left) the scanned position and the nominal and (right) the two “nested” collection boxes used to check that extraction is in the far-field regime.

(a) Map of coupled PIROW guides with attempt to implement PT symmetry onto the left one with the same variable added gain g in the inverse ridge and in the shaded region above it, (b) real part of neff, and (c) imaginary part of neff

Exceptional point restoration: by adding complex gain (i.e., simultaneous gain and index change) in the region above the inverse-rib [see Fig. 9(a)], a nearly perfect singularity in the vicinity of the exceptional point can be restored. (a) Real part of neff and (b) imaginary part of neff.